Sequential PROPHET-600 - CASSETTE INTERFACE; 8-1 RECORDER AND TAPE SELECTION

The POT MUX has its own analog power supply. ♦ 15V is supplied through TB101. IJI09
is a 1% precision 5V regulator which gaurantees that the POT MUX reference voltage
is at least 4.95V. The wheels are operated from the ♦ 15V line. Pull-ups R101 and R102
insure that the wipers cross through the 5V point, so they are fully digitized. DI23 and
0124 clamp the PITCH and MOD wheel voltages to not exceed the tolerance of U108.
For example, when it is the MOD wheel's turn to be read, the CPU (through UI07) sets
POT MUX address lines PM I, PM2 and -PENO high, and the rest low. These select the
Y6 input on both 4067s, but enable only U108. Vmux now equals the MOD wheel
voltage sent across TB10I to the ADC on PCB 4.
See Sheet E. Vmux is applied to the non-inverting input of U419. The output of the
comparator is polled by the miscellaneous input driver, U325, (-CSI0, see Sheet D,
SD600-3 2/2). The system performs a seven-bit successive approximation routine to
convert the current wheel or knob position to a digital number (the PITCH wheel is
digitized to eight bits). This is done by first setting the most significant (seventh) DAC
bit and sampling the comparator output. If the comparator output is high, then the
DAC voltage did not exceed the wheel voltage, so that bit is left set. It then tests
each bit and leaves each one set which does not cause the comparator output to go
low. This takes seven (or eight) separate output operations. (Refer to Figure 2-2.)
Because the front panel operates from a minimum 4.95V and the DAC is set to 4.9V,
the pots are gauranteed to have maximum conversion range (111 1111). It is therefore
normal for pots to "max out" as low as S on the dial.
2-13 THE SYNTHESIZER
How the microcomputer system processes the controls into CVs and switch bits has
been described. The synthesizer converts these control signals into audio.
Each of six voices contains two CEM 3340 voltage-controlled oscillators (VCOs), a few
analog switches and voltage-con trolled amplifiers (VCAs), and a CEM 3372. The voices
have been considerably simplified by the integration of voltage-controlled mixer,
filter, and amplifier functions into this IC.
The VCOs, such as U42S OSC 1A (see sheet F), operate in exponential (as opposed to
linear) control mode, scaled at 1/2 volt per octave. This means an OSC FREQ CV
change of 1/2V produces a pitch change of one octave, and is a departure from our
previous practice of using lV/octave. Doubling the sensitivity of the oscillators allows
the basically 5-V DAC to still control a ten (instead of five) -octave range. Actually
OSC A and B range nine octaves, following up to 2.5V (five octaves) provided by the
keyboard and up to 2V (four octaves) provided by the FREQUENCY knobs.
The VCO itself is of course a complex of other circuits. Basically, the summed CV
drives an exponential control current generator, which charges an external timing
capacitor. The increasing positive charge produces the ascending portion of the
triangle wave by direct integration. When this charge reaches a specific level, a
comparator switches-in a discharge path for the capacitor, producing the descending
portion of the triangle wave. The sawtooth is obtained from the triangle via another
comparator and switch. The pulse wave is created by a comparator which toggles as
the sawtooth level matches the pulse width CV input.
TM600A 7/S3
2-15